Dynamical Collapse in W51 Massive Cores: CS (3–2) and CH3CN Observations

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Abstract
We present interferometric observations of the W51 region at 2 mm with the Nobeyama Millimeter Array. The 320 MHz band centered at 147 GHz covers transitions of the CS (3-2), CH3CN (8-7), H35α, CH3OCH3 (7-6) and (6-6), and HCOOCH3 (12-11), as well as the 2 mm continuum. Toward two dense cores, W51e2 and W51e8, spectroscopic signatures of cloud collapse are present in all the molecular lines observed. Line asymmetries increase systematically toward transitions of larger optical depths, consistent with expected signatures of a centrally condensed infalling cloud. Furthermore, the disparity between the blueshifted and the redshifted emission of the CS line is enhanced when the line is synthesized at higher angular resolution. Given that the continuum source is embedded in the core, we are able to locate the emitting gas of the blueshifted emission on the rear side and the gas of the redshifted emission on the front side of the central star. The new observations confirm our infall interpretations regarding the e2 and e8 cores based on the NH3 data. The compact CH3CN emission allows us to identify velocity gradients in the e2 and e8 cores. The gradient in the e2 core is consistent with the spin-up motions proposed by Zhang & Ho in 1997. CH3CN emission reveals hot components in the W51e2, W51e8, and W51-North:dust cores. The rotational temperature in each core is greater than 100 K. In W51-North, we detected a dust peak coincident with the peak of the dense molecular core. There is no 3.6 cm continuum detected at a level of less than 1 mJy. Although H2O and OH masers in the neighborhood indicate outflow activities, high-velocity gas is not apparent in the CS (3-2). This source may represent an extremely early evolutionary stage: the phase of massive protostars.
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